The paper web deforms geometrically during manufacture, or during subsequent processing operations, especially operations involving wetting or drying of the paper web. This deformation includes shrinking as its water content falls below about 35% by mass, and stretching or elongation due to applied forces. Forces are applied especially in the direction of movement of the web, including a tension in the plane of the web. The tension is generally not uniform, since the shrinkage and elongation are not uniform, and these three properties are related to each other. However, shrinkage and elongation are properties of the paper web, while tension is a property of the process. Nonuniformity in the web tension leads to problems in operating the paper making or paper processing equipment, including web breaks which cause production interruptions. Nonuniformity in the elongation leads to a variety of problems, including sheet breaks, caused by the fact that the paper web is longer in some place than in others.
U.S. Pat. No. 5,694,448 discloses a solution for regulating the web tension profile. That method requires measurement of the caliper and grammage profiles in addition to the web tension profile. Further, it achieves its aim of controlling the web tension profile by manipulating the actuators which are also used to control the profiles of caliper and grammage. Thus, improvements in the web tension profile are generally achieved at the cost of effects in the caliper and grammage profiles, and a compromise solution is obtained. Further more, the solution enables the regulation of the web tension profile only, and does not consider the web elongation profile.
An article "Optimize or compromise? The art of former section tuning". Odell M. 51 Appita Annual General Conference 1997 Proceedings, Volume 1 discloses controlling the web tension profile by using a steam box in the press section to adjust the shape of the moisture profile going into the dryers. By making the edges drier and thus changing the drying rate profile it is possible to reduce the tension profile error at the reel. The moisture profile at the reel is corrected using rewetting in the dryers. In this case too, only the web tension is adjusted, and does not consider the web elongation profile. In failing to consider the web elongation, control of the web tension can lead to an inferior solution with undesirable effects on the web elongation profile.